On the one hand, of course it is, because Macs are slow at running Linux stuff in the same way that Linux is slow at running non-Linux stuff.

On the other hand, Apple should decide if they care about Docker performance. The answer seems to be "a little" (Hypervisor.framework) but much less than, say, Microsoft.

Apple doesn't talk about their future plans. Today we see stagnation, YET with spikes of exotic ideas (e.g. L4, which would permit efficient L4 Linux).

Per Apple's style, a big kernel change on the Mac side would absolutely be tied to a hardware change, to break things once and not twice. Build a new Mac with a Linux-friendly kernel (perhaps Linux, perhaps modified L4, or something new), put it on their beastly ARM CPUs, and I'm drooling.